1. Field of the Invention
The present invention relates to a ceramic femur head for use in an artificial joint such as an artificial hip joint (coxa), artificial shoulder joint or the like for human being. The present invention further relates a femur system using such a ceramic femur head.
2. Description of the Related Art
An example of a prior art artificial hip joint is shown in FIG. 5. In FIG. 5, an artificial hip joint is generally designated by 100 and shown as including a femur head 101 constituting part of a sliding joint, a metallic stem 102 embedded in a femur or thighbone to attach the femur head 101 to a hipbone side end F of the thighbone and a socket 103 made of a high density polyethylene (hereinafter referred to as PE) and fixed to a highbone M side. The femur head 101 is slidably received in the socket 103.
The femur head 101 has a sufficient strength for practical use and thus has been used to effect a good result but has a problem of being relatively smaller in strength as compared with a metallic femur head.
As a countermeasure for solving this problem, a study has been made for applying a method of increasing the retaining or holding strength in a tapered fit system (i.e., a kind of press fitting in which joining surfaces are tapered correspondingly) which is frequently seen in machine tools, to an artificial joint. That is, in machine tools, it is known that the holding or retaining strength become larger as the finish of the fitting or joining surface becomes higher, so it is considered desirable in the artificial joint to make the degree of finish as high as possible i.e., make the surface roughness as small as possible (so long as the cost permits).
For this reason, the tapered portion 102a of the metallic stem 102 and the tapered hole 101a of the ceramic femur head 101 are subjected to surface grinding or the like so as to have a higher degree of surface finish, i.e., a smaller surface roughness. In this instance, due to the complicated shape, the tapered portion 102a of the metallic stem 102 is generally lower in the degree of surface finish than the tapered hole 101a of the ceramic femur head 101.
However, the femur head 101 having fitted therein the metallic stem 102 reduces markedly in the fracture strength as it reduces in size, i.e., in diameter, notwithstanding the above described countermeasure, so it has been difficult to make such a ceramic femur head 101 that has a sufficiently large strength for its size.